Since group 11-13 and group 11-16 compound semiconductors having the chalcopyrite crystal structure, and group 12-16 compound semiconductors having the cubic or hexagonal crystal structure have large absorption coefficients for light having a wavelength within a range of from the visible region to the near-infrared, such compound semiconductors are expected as materials for highly-efficient thin-film solar cells. Representative examples of such compound semiconductors include Cu(In,Ga)Se2 (hereinafter also referred to as “CIGS” or “Cu—In—Ga—Se”) and CdTe.
In the field of CIGS thin-film solar cells, soda-lime glass has been used as a substrate to obtain a solar cell since it is inexpensive and its thermal expansion coefficient is close to that of a CIGS compound semiconductor.
Further, for the purpose of obtaining a solar cell having high efficiency, a glass material having resistance to a high temperature for heat treatment has also been proposed (Patent Document 1).